Inflammation plays an essential role in vascular injury in atherosclerosis and restenosis. Adhesive interactions between vascular cells orchestrate this inflammatory response. We have focused on the leukocyte integrin Mac-1 (CD11b/CD18), identifying this adhesive receptor as a molecular determinant of neointimal thickening after arterial injury. This led us to hypothesize that "outside-in" signaling by Mac-1 may initiate a gene program that promotes vascular inflammation. We have cloned a novel forkhead transcription factor (Mac-l-regulated forkhead, MFH) using differential display PCR that is down-regulated in Mac-1 clustered compared to non-clustered monocytic cells. MFH functions as a transcriptional repressor of genes known to regulate monocyte differentiation, including PU.1 and c-fms, and over-expressing MFH prevents macrophage-like differentiation in vitro. The central hypothesis of this proposal is that Mac-1 engagement orchestrates monocyte differentiation signals by regulating the expression of a novel forkhead transcription repressor, thereby modulating vascular inflammation and ultimately neointimal thickening after injury. Our specific aims are: (1) to define the molecular mechanism of MFH's ability to modulate transcriptional activity and to characterize the nature of MFH regulation by Mac-1; (2) to determine the effect of manipulating MFH expression on monocyte differentiation in vitro and in vivo; and (3) to examine the effect of retroviral bone marrow stem cell over-expression of MFH on macrophage accumulation and neointimal thickening after arterial injury. The experiments outlined in this proposal should clarify the role of Mac-1 signaling in monocyte differentiation and its importance in vascular injury. Understanding the molecular mechanisms of monocyte differentiation will provide insights necessary to develop anti-inflammatory strategies for modulating vascular injury in atherosclerosis, restenosis, and transplant arteriopathy.